We describe the design of the Managing Anti-coagulation Services Trial (MAST), a practice-improvement trial testing whether anticoagulation services are a preferred method of managing anticoagulation for stroke prevention among patients with atrial fibrillation. Most randomized trials within the health care environment are designed as efficacy studies to determine what works under ideal conditions or ideal clinical practice. In contrast, effectiveness trials seek to generalize the results of efficacy studies by determining what works under more typical practice conditions. Practice-improvement trials are effectiveness trials that examine the management of a clinical problem in the context in which care is usually given. Noteworthy features of the MAST include defining the intervention in functional terms and collaboration with managed care organizations.